In the oil and gas and some other industries there are a lot of contaminated products containing elemental sulphur and contaminants such as fine clays, sand, pebbles and gravel, and other inorganic material, as well as organic material, such as humus, wood, leaves and other vegetation. It is important to recover the elemental sulphur from its contaminants. If this is effectively done, the elemental sulphur can be recovered and the environment be improved.
The process of elemental sulphur recovery from contaminated products has been investigated for several years without full commercial and technical success. Basically, hot processes are used to melt the contaminated elemental sulphur and filter or separate by gravity to remove the contaminants from the melted elemental sulphur.
The hot remelt and filtration processes have drawbacks in treating contaminated elemental sulphur materials. Inorganic contaminants cause fouling of heat transfer surfaces in the melting process. This results in lower efficiency and higher operating costs. Also, this process produces a waste product containing up to 85% elemental sulphur. In the melting process, organic contaminants adversely affect the recovery of a high elemental sulphur melt product. The most important organic contaminant is carsul which is a long chain carbon-sulphur compound formed when organic substances come into close contact with molten elemental sulphur and could result in various operating difficulties, such as fouling the process equipment and plugging the filter surfaces.
At Canterra Energy Ltd.'s Ram River gas plant a hot contaminated elemental sulphur recovery system has been studied, constructed, operated, evaluated and shut down. This system had operational problems due to fouling of the heat transfer surfaces. The maximum remelt rate achieved was 2.8 tonnes per hour over a two hour period. The filter screens required cleaning on a continual basis. The system produced an unprocessable by-product in the form of a complex sulphur agglomerate. The sulphur content of this by-product was analyzed by combustion analysis and found to contain 40-60% sulphur. The process was discontinued when operating costs could not be lowered to less than the economic threshold.
There are developments for hot processing systems which have improved waste handling methods and have increased the size of the units. However, the production of high elemental sulphur content waste products still exists.
All hot processes have the disadvantage of producing organic combinations with elemental sulphur which are objectionable and difficult to minimize or eliminate. Also, because all these processes are carried out in a hot environment, they create objectionable environmental problems.
Additionally, non-hot remelting processes have been investigated, such as:
solvent extraction in which elemental sulphur is taken into solution with a solvent; PA1 burning the contaminated elemental sulphur to SO.sub.2 for injection to a Claus recovery plant; however, the contaminant combustion products could adversely affect the recovery plant catalyst; PA1 use of two immiscible liquids which differentiate between elemental sulphur and its contaminants by differences in density and wettability. PA1 grinding the ore to a size sufficiently fine to physically separate the valuable minerals from one another and from the adhering undesired minerals or dirt; PA1 making conditions favourable for the adherence of the minerals of interest to air bubbles; PA1 creating a rising current of air bubbles in the ore slurry; PA1 forming a mineral-laden froth on the surface of the slurry; and PA1 skimming off the mineral-laden froth. PA1 Elemental sulphur is a naturally flotable material, mainly due to its non-wetting nature and high luster. This advantage plus the fact that contaminated elemental sulphur material is friable, as determined by laboratory experiments, provides an ideal application for flotation; PA1 The contaminated elemental sulphur material, being friable, requires minimal size reduction apparatus for crushing and grinding; also, abrasion of the equipment is low due to the friability and softness of the material treated; this situation is advantageous compared to other mining applications using froth flotation where abrasion, due to the hardness of the material treated, is an important factor and causes more frequent parts replacement and higher operational costs; PA1 The elemental sulphur has been found to be readily flotable in acid or alkaline slurries without requiring pH control. This is advantageous, eliminating the use of agents for pH control. This fact differentiates the flotation of the contaminated elemental sulphur from the flotation processes applied in the non-ferrous and ferrous mineral mining industry, where pH control is usually necessary for satisfactory flotation results; PA1 A major portion of the organics will be rejected in the flotation tailings and thus very little of the organics will follow the elemental sulphur flotation concentrate; PA1 The froth flotation process can produce high recovery and high purity elemental sulphur from very contaminated elemental sulphur or from complex sulphur agglomerate, reject by-product resulting from hot melting processes. This differentiates the froth flotation process from the hot remelt processes because the latter could have processing difficulties when the impurities are in significant quantities and typically, when the percentage of fines is high; PA1 Froth flotation is a "cold" process (compared to melting processes) eliminating environmental dangers, such as: PA1 The reagent quantities required for elemental sulphur recovery by froth flotation are minimal, for example, in the order of 0.08-0.73 pounds of frother reagent per ton of material treated. In a specific embodiment, the frother used is methyl isobutyl carbinol (MIBC) or other alcohol frothers or their combinations in the range of quantities mentioned above. The promoter/collector reagent, such as kerosene or fuel oil in small amounts (in the order of 0.05 to 0.44 pounds per ton of material treated) can assist in "coarse" flotation of elemental sulphur (finer than 10 mesh particles with a substantial amount of larger than 48 mesh particles in the slurry). If the slurry has finely ground components (for example finer than 48 mesh and specifically finer than 100/200 mesh) the kerosene or fuel oil may be not necessary. Also, the quantity of regulating/dispersing reagent, if it is used as a slime dispersant, is minimal or may even be eliminated. In specific embodiments wherein "fine" flotation of the complex sulphur agglomerate, reject by-product resulting from hot melting processes is utilized, the slime dispersant used is sodium silicate in the range of 2-5 pounds per ton of material treated. Other slime dispersants known by specialists in this field could also be used. Lime, in the range of 1-10 pounds per ton of dry material treated may be used for minimizing corrosion of the flotation equipment, depending on the acidity of the slurry; PA1 The number of classes of reagents for contaminated elemental sulphur flotation is flexible and can be cut down to three (frother, promoter/collector and regulating/dispersing reagents) or two (frother and promoter/collector reagents or frother and regulating/dispersing reagents) or one (frother reagents), depending primarily on the characteristic of the slurry. This is in contrast with the nonferrous mineral mining industry, which represents the largest user of flotation processes, where typically more classes of reagents are used; PA1 The froth flotation installation could be designed and built in modular form which could be transportable, giving flexibility for moving to other sites. PA1 Step 1: The calculation of the metallurgical balance for a specific product by the formula: ##EQU2## Step 2: The calculation of the elemental sulphur recovered for a specific product by the formula: ##EQU3## (i.e. the numerator is the result of Step 1 and the denominator is the summation of all the specific products as in Step 1.) PA1 Step 1: The elemental sulphur metallurgical balance for the final flotation concentrate equals 76.3224, calculated as follows: ##EQU4## Step 2: The elemental sulphur recovered in the final concentrate equals 98.5%, calculated as follows: ##EQU5## PA1 after grinding and dilution to 20% solids was 5.24. PA1 after flotation was 6.38. PA1 of the tap water was 7.20. PA1 The grinding time was 30 minutes, obtaining particle sizes in the order of finer than 100 mesh; the pH of the slurry at 20% solids was 4.91; pH of the tap water was 7.23. PA1 The Rougher flotation froth was heated to 180.degree. F. by injecting live steam into the slurry for 5 minutes before cleaning 3 times by reflotation. MIBC was replaced by kerosene in the Scavenger stage. The pH of tailings after Rougher and Scavenger flotations was 5.36. PA1 The grinding was done with ceramic instead of steel balls resulting in a fineness of less than 35 mesh. The amount of lime added to the mill was 3 lbs./ton of material treated (2.25 g). PA1 In the Rougher flotation stage, only MIBC frother was used. This also applied to the following two cleaning stages. PA1 Since considerable amounts of coarse elemental sulphur did not float with MIBC only, a Scavenger froth was created using kerosene and MIBC. PA1 pH of the flotation input=7.60 PA1 pH of the flotation tails=7.57 PA1 Attrition scrubbing is a viable alternative to ball milling. PA1 All coarse elemental sulphur (finer than 10 mesh) floated. PA1 Total elemental sulphur recovery, including half of the Midds, will be 96.03% with a purity of 98.4%. PA1 The Rougher flotation time was reduced to 7.5 minutes because there were more granular particles in the slurry which responded faster to flotation. PA1 in the coarsest fraction (+14 mesh) and in the next coarsest fraction (-14 to +20 mesh) of the tailings almost all the elemental sulphur was floated (0.01% unrecovered elemental sulphur); PA1 the least coarse fraction (-20 mesh) of the tailings contains the most elemental sulphur (3.01% unrecovered elemental sulphur). PA1 The triple cleaned elemental sulphur flotation concentrate contained 3.12% by weight particles larger than 325 mesh. This oversize fraction analyzed 89.42% elemental sulphur purity. The minus 325 mesh fraction had a calculated analysis of 88.78% elemental sulphur purity. PA1 The third stage of cleaning appears to be more beneficial than grinding to a finer size in producing maximum product purity. Note in Test No. 6 the purity after two stages of cleaning was 86.8% elemental sulphur. PA1 The total elemental sulphur recovery, which will include half of the elemental sulphur content of the Midds, in a continuous plant operation, will be 96.67% with a purity of 88.8%. PA1 first stage=3.88. PA1 second stage=4.09. PA1 third stage=4.42. PA1 Grinding time was 60 minutes instead of 45 minutes in order to reduce the material fineness to less than 325 mesh. The pH of the tap water was 7.25. PA1 In the third cleaning stage, the elemental sulphur froth for the first and second 21/2 minute flotation periods was kept separate and analyzed to determine the elemental sulphur purity and recovery after the first and after the second period. PA1 first stage=3.88. PA1 second stage=4.04. PA1 third stage=4.39. PA1 A screen analysis of No. 1 and No. 2 combined concentrates showed that 6.38% of the material was larger than 325 mesh. The analysis of this fraction showed 10.82% ash and 89.18% elemental sulphur purity. This is slightly higher than the 87.95% elemental sulphur purity in the combined total concentrate. PA1 The test shows some of the sulphur is recoverable at a purity over 90% sulphur. The sample analysis of the flotation concentrate No. 1, which was taken during the first 21/2 minutes of the third cleaner stage, resulted in 91.8% elemental sulphur purity with a recovery of 43.03%. PA1 95% sample No. 500 - contaminated elemental sulphur base pad PA1 5% sample No. 301 - complex sulphur agglomerate, reject by-product from hot melting processes PA1 contaminated elemental sulphur, PA1 complex sulphur agglomerate, reject by-product from hot melting processes, PA1 a.sub.1 - is the fraction of complex sulphur agglomerate, reject by-product from hot melting processes intended to be used in combined plant feed. PA1 a.sub.2 - the fraction of contaminated elemental sulphur intended to be used in combined plant feed. PA1 b.sub.1 - elemental sulphur purity obtainable from complex sulphur agglomerate, reject by-product from hot melting processes alone (%). PA1 b.sub.2 - elemental sulphur purity obtainable from contaminated elemental sulphur (%). PA1 c.sub.1 - contribution in elemental sulphur purity of the complex sulphur agglomerate, reject by-product from hot melting processes intended to be used in combined plant feed (%). PA1 c.sub.2 - contribution in elemental sulphur purity of the contaminated elemental sulphur intended to be used in combined plant feed (%) . PA1 c.sub.3 - overall elemental sulphur purity of intended composite plant feed after taking into account unavoidable process losses in elemental sulphur purity up to 2%.
The above-mentioned non-hot melting processes have not yet been developed and commercially applied in the oil and gas industry.
In this disclosure a "cold" process (as compared to melting processes) is presented for the recovery of elemental sulphur from contaminated elemental sulphur products existing in the oil and gas industry and in other industries with contaminated elemental sulphur products, which process eliminates the above-mentioned adverse factors related to hot processes and provides a higher recovery of elemental sulphur. This process, which is froth flotation, uses reagents to recover elemental sulphur as a high purity product from an aerated water and solids slurry. The solids are ground to a sufficient fineness which physically frees the elemental sulphur from the contaminants. The contaminants are primarily inorganic materials, such as fine clays, sand, pebbles, and gravel, but some organic materials are also present, such as humus, wood, leaves and other vegetation. The quantity of reagents required is very low and the reagents used are generally not objectionable from an environmental standpoint.